FIG. 1 illustrates a circuit diagram of a conventional DC to DC converter 100. The DC to DC converter 100 includes a transformer 102 and a switch SW0 coupled in series with the primary winding of the transformer 102. A control signal 106 controls the switch SW0 to control the output power of the DC to DC converter 100. By way of example, the control signal 106 can turn on the switch SW0 to couple the primary winding of the transformer 102 with a power source (e.g., the DC voltage VDC), such that a primary current IP flows through the primary winding of the transformer 102. Accordingly, a secondary current IS flows through the secondary winding of the transformer 102 and flows through the inductor L to the output terminal of the DC to DC converter 100. Meanwhile, the inductor L stores magnetic energy. The control signal 106 can also turn off the switch SW0 to decouple the primary winding from the power source, such that the primary current IP is cut off. Meanwhile, the inductor L discharges power to the output terminal of the DC to DC converter 100 by transforming the magnetic energy into electrical energy. The control signal 106 can increase the output of the DC to DC converter 100 by increasing the duty cycle of the switch SW0, or decrease the output of the DC to DC converter 100 by decreasing the duty cycle of the switch SW0.
When the primary current IP is within a specified range, e.g., |IP|<ISPEC, the magnetic flux density 104 of the transformer 102 can be linearly proportional to the primary current IP. As such, an amount of power that is transferred from the primary winding to the secondary winding can be controlled by the primary current IP. However, due to the inherent nature of transformers, if the primary current IP exceeds a non-saturation range, e.g., |IP|>ISATU, the magnetic flux density 104 of the transformer 102 remains substantially unchanged. The threshold ISATU of the non-saturation range is greater than the threshold ISPEC of the specified range mentioned above. Thus, the primary current IP may not be able to control the power transfer of the transformer 102 if the primary current IP exceeds the non-saturation range.
In the DC to DC converter 100, the control signal 106 turns on the switch SW0 at a constant frequency. On one hand, if the DC to DC converter 100 powers a heavy load, the control signal 106 can increase the duty cycle of the switch SW0 such that the DC to DC converter 100 provides enough power to the heavy load. “A “heavy load” as used herein means a load that consumes relatively high power compared to a “light load.” Disadvantageously, when the duty cycle of the switch SW0 is greater than a duty cycle threshold, the primary current IP exceeds the non-saturation range of the transformer 102 and the power transfer of the transformer 102 may not be controlled properly. On the other hand, if the DC to DC converter 100 powers a light load, the control signal 106 can decrease the duty cycle of the switch SW0. A “light load” as used herein means a load that consumes relatively low power compared to a heavy load. However, since the DC to DC converter 100 performs the switching-on operations on the switch SW0 at a constant frequency, the power efficiency of the DC to DC converter 100 is relatively low when the DC to DC converter 100 powers a light load.
FIG. 2A illustrates a circuit diagram of another conventional DC to DC converter 200. The DC to DC converter 200 is an LLC (inductor-inductor-capacitor) resonant converter. The DC to DC converter 200 provides output power to a load 214. As shown in FIG. 2A, the DC to DC converter 200 includes a pair of switches SW1 and SW2, a capacitor 202, an inductor 204, a transformer 208, and a rectifier 212. The inductor 210 represents an equivalent inductor of the primary winding of the transformer 208. A pulse-width modulation (PWM) signal 206 having a 50% duty cycle turns on the switches SW1 and SW2 alternately such that an oscillating current IOSC flows through the capacitor 202, the inductor 204, and the inductor 210. The PWM signal 206 can control the output power of the DC to DC converter 200 by controlling a switching frequency f206 of the switches SW1 and SW2.
More specifically, the DC to DC converter 200 has a resonance frequency fR that is determined by the capacitor 202, the inductor 204, the transformer 208, and the load 214. The PWM signal 206 can control the switching frequency f206 of the switches SW1 and SW2 to be close to the resonance frequency fR so that the DC to DC converter 200 provides more power to the load 214, or the PWM signal 206 can control the switching frequency f206 to be away from the resonance frequency fR so that the DC to DC converter 200 provides less power to the load 214.
However, according to the inherent nature of LLC resonance converters, if the load 214 is a light load, the variation rate of the output voltage VOUT versus the switching frequency f206 is either too high or too low. By way of example, FIG. 2B illustrates a relation diagram of the output voltage VOUT versus the switching frequency f206 when the DC to DC converter 200 powers a light load. As shown in FIG. 2B, when the switching frequency f206 is less than a specified frequency fN1, the variation rate of the output voltage VOUT versus the switching frequency f206 is relatively high, and the output voltage VOUT may be unstable. When the switching frequency f206 is greater than the specified frequency fN1, the output voltage VOUT approaches a limit VLM as the switching frequency f206 increases. Consequently, the DC to DC converter 200 may not be able to control the output voltage VOUT properly.